Spring Semester, 2003
Aspects & Observer Pattern
© 2003, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 06-Feb-03
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CS 683 Emerging Technologies Spring Semester, 2003 Aspects & Observer Pattern |
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© 2003, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 06-Feb-03 |
public class Subject {
Window display;
public void someMethod() {
this.modifyMyStateSomeHow();
display.addText( this.text() );
}
}
public class Subject {
ArrayList observers = new ArrayList();
public void someMethod() {
this.modifyMyStateSomeHow();
changed();
}
private void changed() {
Iterator needsUpdate = observers.iterator();
while (needsUpdate.hasNext() )
needsUpdate.next().update( this );
}
}
public class SampleWindow {
public void update(Object subject) {
text = ((Subject) subject).getText();
etc.
}
}
public interface Observer {
public void update(Subject s);
}
public interface Subject {
public void attach(Observer o);
public void detach(Observer o);
public void notifyObservers();
}
import java.awt.Color;
import java.util.HashSet;
import java.util.Iterator;
public class Point implements Subject {
private HashSet observers;
private int x;
private int y;
private Color color;
public Point(int x, int y, Color color) {
this.x=x;
this.y=y;
this.color=color;
this.observers = new HashSet();
}
public int getX() { return x; }
public int getY() { return y; }
public void setX(int x) {
this.x=x;
notifyObservers();
}
public void setY(int y) {
this.y=y;
notifyObservers();
}
public Color getColor() { return color; }
public void setColor(Color color) {
this.color=color;
notifyObservers();
}
public void attach(Observer o) {
this.observers.add(o);
}
public void detach(Observer o) {
this.observers.remove(o);
}
public void notifyObservers() {
for (Iterator e = observers.iterator() ; e.hasNext() ;) {
((Observer)e.next()).update(this);
}
}
}
import java.util.HashSet;
import java.util.Iterator;
public class Screen implements Subject, Observer {
private HashSet observers;
private String name;
public Screen(String s) {
this.name = s;
observers = new HashSet();
}
public void display (String s) {
System.out.println(name + ": " + s);
notifyObservers();
}
public void attach(Observer o) {
this.observers.add(o);
}
public void detach(Observer o) {
this.observers.remove(o);
}
public void notifyObservers() {
for (Iterator e = observers.iterator() ; e.hasNext() ;) {
((Observer)e.next()).update(this);
}
}
public void update(Subject s) {
display("update received from a "+s.getClass().getName()+
" object");
}
}
public static void main(String argv[]) {
Point p = new Point(5, 5, Color.blue);
System.out.println("Creating Screen s1,s2,s3,s4,s5 and Point p");
Screen s1 = new Screen("s1");
Screen s2 = new Screen("s2");
Screen s3 = new Screen("s3");
Screen s4 = new Screen("s4");
Screen s5 = new Screen("s5");
p.attach(s1);
p.attach(s2);
p.attach(s3);
p.attach(s4);
s2.attach(s5);
s4.attach(s5);
p.setColor(Color.red);
p.setX(4);
}
import java.awt.Color;
public class Point implements Subject {
private int x;
private int y;
private Color color;
public Point(int x, int y, Color color) {
this.x=x;
this.y=y;
this.color=color;
}
public int getX() { return x; }
public int getY() { return y; }
public void setX(int x) { this.x=x; }
public void setY(int y) { this.y=y; }
public Color getColor() { return color; }
public void setColor(Color color) { this.color=color; }
}
public class Screen {
private String name;
public Screen(String s) {
this.name = s;
}
public void display (String s) {
System.out.println(name + ": " + s);
}
}
import java.util.WeakHashMap;
import java.util.List;
import java.util.LinkedList;
import java.util.Iterator;
public abstract aspect ObserverProtocol {
protected interface Subject { }
protected interface Observer { }
private WeakHashMap perSubjectObservers;
protected List getObservers(Subject s) {
if (perSubjectObservers == null) {
perSubjectObservers = new WeakHashMap();
}
List observers = (List)perSubjectObservers.get(s);
if ( observers == null ) {
observers = new LinkedList();
perSubjectObservers.put(s, observers);
}
return observers;
}
public void addObserver(Subject s, Observer o) {
getObservers(s).add(o);
}
public void removeObserver(Subject s, Observer o) {
getObservers(s).remove(o);
}
protected abstract pointcut subjectChange(Subject s);
protected abstract void updateObserver(Subject s, Observer o);
after(Subject s): subjectChange(s) {
Iterator iter = getObservers(s).iterator();
while ( iter.hasNext() ) {
updateObserver(s, ((Observer)iter.next()));
}
}
}
import java.awt.Color;
public aspect ColorObserver extends ObserverProtocol{
declare parents: Point implements Subject;
declare parents: Screen implements Observer;
protected pointcut subjectChange(Subject s):
call(void Point.setColor(Color)) && target(s);
protected void updateObserver(Subject s, Observer o) {
((Screen)o).display("Screen updated because color changed.");
}
}
public aspect CoordinateObserver extends ObserverProtocol{
declare parents: Point implements Subject;
declare parents: Screen implements Observer;
protected pointcut subjectChange(Subject s):
( call(void Point.setX(int)) ||
call(void Point.setY(int)) ) && target(s);
protected void updateObserver(Subject s, Observer o) {
((Screen)o).display("Screen updated as coordinates changed.");
}
}
public static void main(String argv[]) {
Point p = new Point(5, 5, Color.blue);
System.out.println("Creating Screen s1,s2,s3,s4,s5 and Point p");
Screen s1 = new Screen("s1");
Screen s2 = new Screen("s2");
Screen s3 = new Screen("s3");
Screen s4 = new Screen("s4");
Screen s5 = new Screen("s5");
ColorObserver.aspectOf().addObserver(p, s1);
ColorObserver.aspectOf().addObserver(p, s2);
CoordinateObserver.aspectOf().addObserver(p, s3);
CoordinateObserver.aspectOf().addObserver(p, s4);
ScreenObserver.aspectOf().addObserver(s2, s5);
ScreenObserver.aspectOf().addObserver(s4, s5);
p.setColor(Color.red);
p.setX(4);
}
}